Poly(alkylene carbonates) (PACs) have attracted much attention because of their environmental friendly nature, biodegradability, transparency and good gas barrier properties. These polymers, first prepared by Inoue (Inoue, S. et al, Journal of Polymer Science, Polymer Letter Edition 7, 287 (1969)), are readily obtained by copolymerization of an alkylene oxide and CO2. PACs such as Polypropylene Carbonate (PPC) are typically amorphous and biodegradable, but exhibit poor mechanical properties and lack sufficient thermal stability. They are brittle and readily decomposed at moderate temperature, limiting the applications in which they can be useful. To improve their mechanical properties and thermal stability PACs have been blended with further polymers. For example, WO 2011/005664 describes blends of specific PPC based polymers with polyolefins, namely, low density and high-density polyethylene, as well as polypropylene.
These polymers are however not biodegradable. In order to attend the increasing environmental concerns, PACs have been mixed with poly-3-hydroxyalkanoates, namely poly-3-hydroxybutyrate (3-PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (3-PHBHV) and other copolymers of 3-PHB (e.g. poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)-PHBHHx). 3-PHB and its copolymers are biodegradable polyesters that can be obtained by fermentation, readily available from sources that do not compete with food. Their biodegradability makes them an alternative in PAC based blends.
3-PHB and its copolymers are not however entirely satisfactory and have limited applicability. For example, U.S. Pat. No. 6,576,694 blends PAC, namely PPC, with 3-PHB and 3-PHBHV to yield blends of improved stability, but which are opaque and require further improvement in the mechanical properties thereof. This fact is noted by WO 2007/125039, wherein mixtures of PPC and polylactic acid are proposed, for example, in order to obtain transparent mixtures. Alternatively, low amounts of 3-PHBV are proposed.
WO 2012/007857 discloses a mixture of PAC, thermoplastic cellulose (TPC) and 3-PHB or 3-PHBV. No mention is made of the transparency of the resulting blends and according to this document mixtures of PAC with 3-PHB or 3-PHBV perform poorly in terms of strength and ductility with respect to ternary mixtures of PAC, TPC and PHA.
WO 2014/058655 discloses mixtures of different PHA molecules and methods to prepare such mixtures of PHAs. Although it is generically mentioned that the mixtures of PHAs disclosed therein can be combined with other polymers, it does not disclose any specific example of such blends nor any possible advantages which such blends could bring about.
Thus, mixtures of PACs and polyhydroxyalkanoates described to the date (PAC blended with 3-PHB, 3-PHBV or copolymers thereof) provide blends with poor properties. For example, they lack transparency and frequently result in opaque blends. Other mechanical (e.g. Charpy impact) and thermal properties (e.g. thermal stability) also require improvement in order to further expand the usefulness of PACs, which are otherwise interesting materials due to their environmentally friendly nature.